The invention is in the field of transistor amplifier circuits, and relates more particularly to power amplifier circuits suitable for use in high-frequency applications.
One type of amplifier circuit used in high-frequency applications is the Class E power amplifier, in which the active component is used as a switch which turns on and off at the carrier frequency. Class E power amplifiers have been used in wireless communications apparatus and have been designed in both GaAs MESFET and deep-submicron CMOS technology.
In Class E power amplifiers, the signal swing at the drain of the output transistor can typically be three or more times the power supply voltage. This imposes a limitation on the maximum supply voltage that can be used to avoid gate-drain breakdown in MOS transistors. Thus, for example, in a 0.25 micron CMOS process, the nominal supply voltage is 2.5 volts. However, a 2.5 volt Class E amplifier cannot be designed in this process, as the gate breakdown voltage is 6 volts. Taking into account the signal swing at the gate, which is in opposite phase to the signal swing at the drain, conventional techniques limit the maximum allowable power supply voltage in Class E in this process to 1.5 volts. Since the output power in Class E operation is proportional to the square of the power supply voltage, using 1.5 volts instead of 2.5 volts reduces the maximum power output by a factor of 2.7 for a given load value.
Although various prior-art techniques exist for improving circuit performance using series-connected transistors and bootstrapping techniques, as shown in U.S. Pat. Nos. 3,268,827; 4,100,438; 4,284,905; and 4,317,055, these references do not address the issue of how to maximize usable power supply voltage in a Class E amplifier circuit. Accordingly, it would be desirable to have a Class E amplifier circuit in which power output is not limited by operating the output stage at less than the nominal supply voltage due to component breakdown restraints.
It is therefore an object of the invention to provide a Class E amplifier circuit in which the useable power supply voltage is maximized, so that power output is not limited by voltage constraints imposed by component breakdown characteristics.
In accordance with the invention, this object is achieved by a new Class E amplifier circuit in a bootstrapped dual-gate configuration in which a first MOSFET and a second MOSFET are connected in series and coupled between a dc voltage source terminal and a common terminal, with an rf input signal terminal being coupled to a gate electrode of the first MOSFET and a dc control voltage terminal being coupled to a gate electrode of the second MOSFET. In order to provide a bootstrapping effect, a unidirectionally-conducting element is coupled between a drain electrode and the gate electrode of the second MOSFET, and an output of the amplifier circuit is taken from the drain electrode of the second MOSFET.
In a preferred embodiment of the invention, the dc control voltage source terminal is coupled to the gate electrode of the second MOSFET by a resistor, and the rf input signal terminal is coupled to the gate electrode of the first MOSFET by a capacitor.
In a further preferred embodiment of the invention, the unidirectionally-conducting element is a diode-connected MOSFET, which implements the bootstrapping effect.
A bootstrapped dual-gate Class E amplifier circuit in accordance with the present invention offers a significant improvement over prior-art Class E amplifiers in that the useable power supply voltage is maximized to achieve substantially increased power output.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.